20% off Every 3rd Mobile Pre-paid Recharge voucher at Australia Post Shop
Telstra ($30), Optus ($30), Vodafone ($30), Boost ($40).
Not sure if it applies to higher recharge amounts, eg: Telstra $40.
Works out to be a 6.66% effective discount is you spread the savings over three purchases, but still better than nothing until another real 20% discount comes.
I found this at a fully fledged Post Office shop (Glenferrie, VIC), not a LPO (Local Post Office) which are franchisees not owned by Australia Post.
Woolworths mobile is cheaper with 10% off every purchase, using a rewards card. If you use the Optus network that is.